Find out how this system of your house works before your house's pipes are hidden from view.

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Plumbers work with three basic categories of supplies. There are the supply pipes, that deliver the clean water into the house and to the plumbing fixtures, such as the sinks, toilets, and washing machine. There are the waste pipes that drain the water and waste from the fixtures. Lastly, there are the fixtures you see and use.

The Supply Pipes
Starting with the service line from the source – perhaps you have a municipal water supply, maybe a well on your property – the supply pipes extend into the home. The main cold-water pipe, called the trunk line, then divides, sending water both to the water heater and to the many branches that supply the fixtures in the house. The cold water supply is paralleled by a hot-water trunk line and a second set of branches that provide the supply of hot water to the fixtures that require both. As with the blood vessels in the body or the branches of a tree, the pipes step down in size as the extremities of the system are reached.

Supply pipes can be iron, copper, or one of numerous varieties of plastic. Copper and plastic pipes are the rule today, but your plumber will look to the specifications to indicate what your job requires. In general, copper is more durable and expensive, plastic cheaper and easier to install. Whatever the material used, the pressure in the system is maintained at all times, so the pipes must be tightly joined. With copper, the joints are soldered; with plastic they’re fastened with a solvent cement.

The Waste Pipes
The waste system is not pressurized. It is dependent upon gravity to function, so all waste pipes must slope downward, away from the fixtures and toward the sewer or septic tank, dropping at a rate of at least 1/4 inch per horizontal foot.

The waste system must also be vented, which means that the plumbing system will have a pipe that extends upward and vents outdoors, typically through the roof. That will allow the water and waste in the system to flow smoothly out to the sewer or septic tank. If there were no vent, the system would drain slowly or not at all, not unlike a water-filled straw when you seal one end with your finger. Vent pipes also allow septic gases to escape.

Every plumbing fixture must have its own trap, a U-shaped space built into the fixture (in the case of toilets) or in an adjacent pipe. The trap remains filled with water at all times, acting as a barrier to sewer gases that would otherwise rise up through the pipes and enter the living spaces of the house.

The Fixtures
While most of the decisions regarding the pipes will be made for you by the designer, plumber, and the building code, you will determine what plumbing fixtures1 you want. The choices vary tremendously: basic sinks and toilets, for example, can be bought for less than $75, but high-tech designer models can cost many times that. As a result, you will probably let your pocketbook be your guide.

Ask your plumber what models he recommends. He probably has a supplier that stocks one of the major national brands, like American Standard or Kohler. Most manufacturers’ lines start with economy models and step up to fancier designs. You will be able to choose color (keep in mind that today’s cutting-edge palette may look dated tomorrow), style (the range is Euro-moderne to funky neo-Victorian), and from different materials. Unless you are striving for a strong designer look, recognizably traditional designs in understated colors are probably the safest choice.

Putting the Pipes in Place
The usual order of things calls for the plumber to install the waste system first. Its pipes are large and ungainly, and it’s easier to work the smaller supply pipes around the waste pipes. At the locations where the fixtures will later be set, the plumbers will stub off the pipes, capping the ends of the supply lines where they protrude through the floor or wall. By capping off the ends of the pipes, the plumbers will also be able to pressure-test the system to be sure there are no leaks. Plumbers and electricians often work side by side.

(Note that if the foundation of your new structure is simply a slab, much of the plumbing rough-ins will have to have been completed before the slab is poured, along with any portions of the mechanical systems that will be buried there, such as steam or electric lines.)

Once the rough-in has been completed, the plumbing will be inspected, usually by the code enforcement officer. He’ll check to see that pipe sizes meet code (minimums are specified for trunk lines, bath, shower, sink, and other supply lines, as well as for waste lines from individual fixtures). The inspector will also check to be sure the plumbing lines are well supported (all pipes must be supported at least every 4 feet, either with clamps or hangers or some other fixing mechanism).

The plumbing work at this stage probably won’t look very familiar to you. There’ll be no fixtures. You may also have no service to existing fixtures in the house for a time. But rest assured, progress is being made.